There have been a number of polymeric products made to look like natural wood for decking and siding applications. Such products are formed by extrusion and embossing processes, or by injection molding in a pattern mold to simulate a wood grain or pattern. Such products comprise a painted or otherwise “decorated” or printed pattern to simulate wood or other materials, such as marble or natural stone, for example. Such teachings are provided in Franco et al., US2005/0053767; Giacchino, US 2005/0127345; Barre et al., U.S. Pat. No. 5,331,602; Anstadt et al., U.S. Pat. No. 4,141,944; Bosler, U.S. Pat. Nos. 5,906,840; 5,314,325, 6,823,794 and 6,641,384; Cameron et al., U.S. Pat. No. 5,053,176; Dorchester et al., U.S. Pat. Nos. 5,866,054 and 5,869,176; Saloom, U.S. Pat. No. 5,387,381; and Soda et al., U.S. Pat. No. 3,936,518, which are hereby incorporated by reference. Most of these disclosures, other than the Bosler patents, relate to the use of embossing rolls located immediately downstream of the extrusion die. The embossing operation is designed to emboss the surface configuration, or provide ornamentation onto the capstock layer side of a plastic sheet. The embossing rolls apply tension to the sheet of plastic to draw the sheet of plastic down to a particular dimension. Following embossing, the embossed sheet is typically preformed in a die into a rough version of a siding profile. See, for example, Dorchester et al. U.S. Pat. No. 5,869,176, at col. 6, lines 11-26. While siding can be embossed readily with good effect, the high pressure of embossing rolls is ill suited for hollow profiles, such as fence boards and hollow decking planks, which would likely collapse under such pressure.
Multiple hollow fence board products made of thermoplastic materials are available in the market. Present hollow, semi-hollow, thin walled fence boards made of polymer based materials (neat, composite, or with fillers) made in extrusion processes have a surface which is smooth or enhanced by longitudinal, machine- or extrusion-direction texture, lines, ribs, or depressions. Such products do not have the look of natural wood, such as softer areas indicative of environmental wear, or harder areas which are generally more resistant to environmental wear. These hard and soft areas form peaks and valleys on the natural wood board surface following natural wood patterns which do not always line up with the machine- or extrusion-direction of synthetically made materials. Furthermore, hollow, thin wall fence board products currently available in the market exhibit a high surface gloss which reveals the true character of this material, and often make them undesirable on aesthetic grounds.
Continuous and semi-continuous processes for creating patterns on extruded plastic sheets have been used in the building components industry for a number of years. Some prior systems have disclosed rigid linked patterns for forming shaped impressions in an extruded sheet material. Unfortunately, such rigid shaped patterns tend to form unsightly horizontal seams in the material. Other systems have used pattern forms on rotating cylindrical drums. Although these processes are continuous, and do not produce horizontal seams, they often require expensive additional equipment and instrumentation to align the arcuate surface of the pattern with the relatively flat surface of the product, and to avoid, or correct, unwanted bowing of the product.
Because of the limitations on prior continuous processes, some manufacturers have opted for injection or blow molding building products one at a time. While such techniques can provide the desired detail in texture and surface finish, they are generally limited to product sizes of about 4-5 feet in length and provide product thicknesses which are practically limited to greater than about 0.080 inches. This is generally because of the difficulty associated with flowing hot viscous polymer through thin cross-sectional profiles in steel molds. Additionally, because of the known size limitations, the randomness of individual features on the surface of a molded product is limited. This results in only a relatively small number of pattern elements, such as shingles, being molded into the relatively small surface area. When several of these products are aligned side by side on a wall or roof of a building, for example, it is sometimes obvious to see the pattern repeated over and over again. Accordingly, there remains a need for improved vacuum embossing techniques for use in connection with extruded hollow thin wall profile products.